Substrate support with gas feed-through and method

ABSTRACT

A substrate support comprises an electrode and a dielectric layer covering the electrode, the dielectric layer having a surface to receive a substrate. A gas feed-through provides a gas to the surface of the dielectric layer and comprises a conduit extending through one or more of the dielectric layer and electrode. A dielectric insert in the gas feed-through has a passage therein that allows the gas to be flowed therethrough. Two opposing electrically conducting cups are around the passage in the dielectric insert.

CROSS-REFERENCE

[0001] This application is a divisional of U.S. patent application Ser.No. 09/360,351, filed on Jul. 22, 1999, entitled “SUBSTRATE SUPPORT WITHGAS FEED-THROUGH AND METHOD” to Mett, et al., which is incorporatedherein by reference in its entirety.

BACKGROUND

[0002] The present invention relates to a chamber having a gasfeed-through for introducing gas into the chamber.

[0003] In the fabrication of electronic devices, semiconductor,dielectric and conductor materials, such as for example, polysilicon,silicon dioxide, and metal containing layers, are deposited on asubstrate and etched to form features such as patterns of gates, vias,contact holes and interconnect lines. These features are typicallyformed by chemical vapor deposition (CVD), physical vapor deposition(PVD), oxidation and etching processes. For example, in a typicaletching process, a patterned mask of photoresist or oxide hard mask isformed on a deposited layer by photolithographic methods and exposedportions of the deposited layer are etched by an energized halogen gas,such as Cl₂, HBr, and BCl₃. In a typical CVD process, a gas provided inthe chamber is decomposed to deposit a layer on the substrate. In PVDprocesses, a target facing the substrate is sputtered to deposit thetarget material onto the substrate.

[0004] In these processes, gas is supplied to the chamber through a gasfeed-through. For example, the gas feed-through can be used to feedprocess gas into the chamber for processing of the substrate. The gasfeed-through can also be used to feed gas to a surface of a supportbelow the substrate, the gas being useful for enhancing heat transferrates to and from the substrate, and for reducing deposition ofbyproducts on the backside or edges of the substrate. However, when anelectrode in the chamber is electrically biased, for example, toenergize a plasma in the chamber or generate electrostatic forces, theelectric potential applied to the electrode can cause plasma formation,arcing, and glow discharge in the gas passing through the gasfeed-through. Such arcing and glow discharges damage the gasfeed-through and adjacent chamber components or surfaces. Arcing canalso cause catastrophic failure of the gas feed-through upon plasmaignition; randomness (or wide error band) of breakdown voltage across alarge substrate; sensitivity of breakdown voltage to materialimperfections, voids, and gaps at interfaces within the gasfeed-through; and cause the applied voltage at which an ignited gas inthe gas feed-through is extinguished to be more than an order ofmagnitude smaller than the voltage at which it is ignited. All theseeffects impede the efficient processing of substrates or otherworkpieces in the chamber.

[0005] Commonly assigned U.S. patent application Ser. No. 08/965,690,filed on Nov. 6, 1997, entitled “Electrostatic Chuck Having Improved GasConduits” to Weldon, et al., which is incorporated herein by reference,describes a ceramic insert that is positioned in a conduit in a ceramicelectrostatic chuck (which has an electrode that is charged toelectrostatically hold an overlying substrate) to reduce plasmaformation in the conduit. While this device reduces plasma formation inthe conduit it has been observed that plasma formation still oftenresults in the conduit when the voltage applied to the electrode exceedscertain levels. In addition, instantaneous changes in electricalpotential can ionize the gas adjacent to the gas feed-through,particularly when the diameter of the conduit is relatively large andprovides a long mean free path that leads to avalanche breakdown of gasmolecules in the conduit. For example, arcing has been occasionallyobserved at voltages of about 2 KVp and at high frequencies of 13.56MHz.

[0006] Thus there is a need for a gas feed-though that reduces plasmaformation, glow discharges and arcing, during passage of gas through thefeed-through. There is also a need for a chamber that can process asubstrate in a gas while reducing the incidence of plasma formation ofgas in conduits that feed gas into the chamber. There is a further needfor a method of providing gas to a chamber while simultaneously reducingthe incidence of plasma formation in the feed-through.

SUMMARY

[0007] A substrate support comprises an electrode and a dielectric layercovering the electrode, the dielectric layer having a surface to receivea substrate. A gas feed-through provides a gas to the surface of thedielectric layer and comprises a conduit extending through one or moreof the dielectric layer and electrode. A dielectric insert in the gasfeed-through has a passage therein that allows the gas to be flowedtherethrough. Two opposing electrically conducting cups are around thepassage in the dielectric insert.

[0008] A substrate support comprises an electrode and a dielectric layercovering the electrode, the dielectric layer having a surface to receivea substrate. A gas feed-through is capable of providing a gas to thesurface of the dielectric layer and the gas feed-through comprises aconduit extending through one or more of the dielectric layer andelectrode. A dielectric insert in the gas feed-through has a passagetherein that allows the gas to be flowed therethrough. Two opposingmetal-containing shields around the passage in the dielectric insert arecapable of reducing plasma formation in the gas feed-through.

[0009] A method of processing a substrate on a support in a chambercomprises passing a gas through a passage in the support and maintainingan electrical shield around a portion of the passage, whereby anincidence of plasma formation of the gas passing through the passage maybe reduced.

DRAWINGS

[0010] These features, aspects, and advantages of the present inventionwill become better understood with regard to the following description,appended claims, and accompanying drawings which illustrate examples ofthe invention, where:

[0011]FIG. 1 is a schematic sectional view of a chamber according to thepresent invention;

[0012]FIG. 2a is a schematic side view of a gas feed-through accordingto the present invention;

[0013]FIG. 2b is a schematic top view of the gas feed-through of FIG. 2aviewed along section 2 b;

[0014]FIG. 3a is a schematic side view of another embodiment of a gasfeed-through according to the present invention;

[0015]FIG. 3b is a schematic top view of the gas feed-through of FIG. 3aviewed along section 3 b;

[0016]FIG. 4a is a schematic side view of yet another embodiment of thegas feed-through comprising dual gas flow passages;

[0017]FIG. 4b is a schematic top view of the gas feed-through of FIG. 4aviewed along section 4 b;

[0018]FIG. 5 is a graph showing a Paschen plot for hydrogen gas;

[0019]FIG. 6 is a schematic representation of an analytical model usedto determine the breakdown voltage in a gas between conducting planes;

[0020]FIG. 7 is a graph of E_(z)(0,z)/E₀ versus z/a for an analyticalmodel of a gap distance in a conducting plane;

[0021]FIG. 8 is a graph of the peak applied voltage (kV) versus heliumgas pressure (T) for a dual zone porous ceramic gas feed-through with anaspect ratio of −z/2a=1, and a 1 inch spacing between conductors; and

[0022]FIG. 9 is a graph of the peak applied voltage (kV) versus heliumgas pressure (T) for a dual zone porous ceramic gas feed-through with anaspect ratio of −z/2a=0.75, and a 1.4 inch spacing between conductors.

DESCRIPTION

[0023] A chamber 10 according to the present invention is for example,an MxP+OXIDE ETCH chamber, commercially available from Applied MaterialsInc., Santa Clara, Calif., as shown in FIG. 2, and generally describedin commonly assigned U.S. Pat. Nos. 4,842,683 and 5,215,619 to Cheng, etal; and U.S. Pat. No. 4,668,338 to Maydan, et al., all of which areincorporated herein by reference. Such chambers can be used in amulti-chamber integrated process system as for example, described inU.S. Pat. No. 4,951,601 to Maydan, et al., which is also incorporatedherein by reference. The particular embodiment of the chamber 10 shownherein, is suitable for processing of semiconductor substrates 15, isprovided only to illustrate the invention, and should not be used tolimit the scope of the invention.

[0024] During processing, the chamber 10 is evacuated to a low pressureof less than about 500 mTorr, and a substrate 15 is transferred to aplasma zone 20 of the chamber 10 from a load lock transfer chamber (notshown) maintained at vacuum. The substrate 15 is held on a support 18,which optionally comprises a mechanical or electrostatic chuck 25. Atypical electrostatic chuck 25 comprises an electrostatic member 30comprising a dielectric layer 35 having a surface 40 adapted to receivethe substrate 15, and the surface 40 having grooves 45 in which a heattransfer gas, such as helium, is held to control the temperature of thesubstrate 15. The dielectric layer 35 covers an electrode 50—which maybe a single conductor or a plurality of conductors—which is chargeableto electrostatically hold the substrate 15. After the substrate 15 isplaced on the chuck 25, the electrode 50 is electrically biased withrespect to the substrate 15 by an electrode voltage supply and RFgenerator 55 to electrostatically hold the substrate 15. A base 60 belowthe electrostatic chuck 25 supports the chuck, and optionally, is alsoelectrically biased with an RF bias voltage.

[0025] Process gas is introduced into the chamber 10 through a gassupply 65 that includes a first gas supply 70 and one or more gasnozzles 75 terminating in the chamber 10. The gas in the chamber 10 istypically maintained at a low pressure. A plasma is formed in the plasmazone 20 from the gas by applying an RF current to an inductor coil (notshown) encircling the process chamber and/or by applying an RF currentto the electrode 50 in the chamber 10. In etching processes, the plasmais typically capacitively generated by applying an RF voltage to theelectrode 50 (which serves as the cathode electrode) and by electricallygrounding the sidewalls 85 of the chamber 10 to form the other (anode)electrode 50. Alternatively, an RF current is applied to an inductorcoil (not shown) to inductively couple energy into the chamber 10 togenerate the plasma in the plasma zone 20. The frequency of the RFcurrent applied to the electrode 50 or to the inductor coil (not shown)is typically from about 50 KHz to about 60 MHz, and more typically about13.56 MHz.

[0026] The plasma can also be enhanced by electron cyclotron resonancein a magnetically enhanced reactor in which a magnetic field generator90, such as a permanent magnet or electromagnetic coils, provides amagnetic field that increases the density and uniformity of the plasmain the plasma zone 20. Preferably, the magnetic field comprises arotating magnetic field with the axis 195 of the field rotating parallelto the plane of the substrate 15, as described in U.S. Pat. No.4,842,683, issued Jun. 27, 1989, which is incorporated herein byreference. Spent process gas and etchant byproducts are exhausted fromthe chamber 10 through an exhaust system 95 capable of achieving aminimum pressure of about 10⁻³ mTorr in the chamber 10. The exhaustsystem 95 comprises an exhaust conduit 100 that leads to a plurality ofpumps 105, such as roughing and high vacuum pumps, that evacuate the gasin the chamber 10. A throttle valve 110 is provided in the exhaustconduit 100 for controlling the pressure of the gas in the chamber 10.Also, an optical endpoint measurement technique is often to determinecompletion of the etching process by measuring a change in lightemission intensity of a gas species in the chamber 10 or measuring theintensity of light reflected from a layer being processed on thesubstrate 15.

[0027] During processing of the substrate 15, heat transfer gas isprovided to the interface between the substrate 15 and the dielectriclayer 35 of the chuck 25, to enhance heat transfer rates therebetween.The heat transfer gas is provided via gas conduits 115 that extendthrough one or more of the electrode 50 and dielectric layer 35. Asecond gas supply 120 supplies heat transfer gas to the conduits 115 viaa gas supply channel 125. The conduits 115 have one or more outlets 130that deliver the gas to the surface 40 of the chuck 25. The substrate 15covers the edges of the dielectric layer 35 to reduce leakage of heattransfer gas from the edge of the chuck 25. The grooves 45 on thesurface 40 of the dielectric layer 35 are sized and distributed to holdheat transfer gas to heat or cool substantially the entire backside ofthe substrate 15, such as for example, a pattern of intersecting grooves45 radiating across the dielectric layer 35. Preferably, at least oneconduit 115 terminates in one of the grooves 45, and more preferably,the conduits 115 terminate at one or more intersections of the grooves45. Alternative groove patterns are described in, for example, U.S.patent application Ser. No. 08/189,562, entitled “Electrostatic Chuck”by Shamouilian, et al., filed on Jan. 31, 1994, which is incorporatedherein by reference. The gas conduits 115, gas supply channel 125, andgrooves 45 are formed by conventional techniques, such as drilling,boring, or milling. Typically, the heat transfer gas comprises helium orargon which is supplied at a pressure of about 5 to about 30 Torr;however, other gases such as CF₄ can also be used.

[0028]FIG. 1 illustrates a schematic of a gas feed-through 150 toprovide gas to the plasma chamber 10, the gas feed-through comprising adielectric insert 155 having an internal passage 160 for passing gastherethrough, and a surrounding electrically conducting cup 165 thatserves as an electrical shield by enclosing substantially the entirepassage. While the gas feed-through 150 is shown to be in a lowerportion 170 of the chamber 10 that lies below the process zone, the gasfeed-through can be located anywhere in the pathway of a gas from a gassupply 65 to the surface 40 of the chuck 25, including in the sidewall85 of the chamber 10, inside the chuck, or in the base 60 below thechuck. Also, in the example provided herein to illustrate the invention,the gas feed-through 150 is used to supply heat transfer gas to thesurface 40 of a support, and the gas feed-through and support form asubstrate support assembly 18. However, the gas feed-through 150 couldalso be used to supply process gas to the chamber 10 for processing ofthe substrate 15, suitable process gas including gases for etching asubstrate, for depositing material on the substrate by chemical vapordeposition, or for assisting in sputtering material from a target byphysical vapor deposition.

[0029] Referring to FIGS. 2a and 2 b, the dielectric insert 155comprises a gas passage 160 through a dielectric material that has adielectric constant that is sufficiently high to reduce plasma formationin or adjacent to the passage. The dielectric material can be a ceramicmaterial such as Al₂O₃, AlN, SiO₂, Si₃N₄, and mixtures thereof. Morepreferably, the dielectric insert 155 comprises aluminum oxide or amixture of aluminum oxides and silicon oxides, as described below.Alternative dielectric materials include thermoplastic and thermosetpolymers, such as for example, polyimide, polyketone, polycarbonate, andTEFLON (a tetrafluroethylene polymer manufactured by Dupont de NemoursCompany Wilmington, Del.). The breakdown strength of the dielectricmaterial is preferably from about 4 to 40 volts/micron. In this version,the dimensions of the passage 160 are sufficiently small to furtherreduce plasma formation therein (by reducing the mean free path of thegas in the passage) and preferably comprise a diameter equal to or lessthan about 0.4 mm. The passage 160 can extend through the spaces ofinterconnected pores, passageways formed by microcracks, or betweenceramic grains that are separated from another and along their grainboundary regions.

[0030] The breakdown of a gas in a gap (such as within the passage inthe dielectric insert 155 or at an interface of the dielectric insertand surrounding conducting metal structures) obeys a Paschen curve, anexample of which is shown for H₂ in FIG. 5, as described in moredetailed in Von Engle, “Ionized Gases” (Oxford University Press, 1955)which is incorporated herein by reference. This empirical curve 175shows the breakdown voltage of the gas as a function of gas pressuretimes a gap width d of the gap. In the experiment that was performed todetermine this portion 170, the distance d represents the gap betweentwo clean smooth conducting plates 190 across which the voltage isapplied. In practice, this distance represents a maximum open distanceparallel to an applied electric field. A counterintuitive but essentialfeature of the Paschen curve is that the breakdown voltage of a deviceis increased at any pressure simply by decreasing the gap distance d.This is because lowering d lowers the breakdown voltage across the gapor void for a given applied voltage across the device. In other words,in a fixed electric field, the breakdown voltage across a gap becomessmaller as the gap itself becomes smaller, and according to the Paschencurve, this increases the breakdown voltage of the device.

[0031] In one aspect of the present invention, an electricallyconducting cup 165 is positioned around at least a portion of thepassage 160. The electrically conducting cup 165 is shaped to reduceplasma formation in the gas feed-through 150. By “electricallyconducting cup” it is meant a cup comprising metal-containing material,such as aluminum, copper, platinum, or metal silicides, such as WSix.The shield comprises a jacket 180 encircling the passage 160 in thedielectric insert 155 of the gas feed-through 150. The jacket 180comprises sidewalls 185 that enclose the passage 160. For a passage 160that is essentially tubular, such as a bored hole, the sidewalls 185conform to and follow the shape of the hole. The sidewalls 185 terminateat endplates on either end of the passage 160, the entire structureforming a cup-shaped shield that surrounds the passage 160. Two opposingcup-shaped shields are oriented to face one another and are centeredalong an axis 195 of the passage 160. These cup-shaped shields arecylindrical in shape and have an aspect ratio of width to height that issufficiently low to reduce plasma formation in the gas feed-through 150.The reduction of the electric field passing through the passage 160depends on the aspect ratio of the electrically conducting cups 165. Toreduce the electric field strength, preferably, the aspect ratio of thewidth to height is preferably from about ⅓ to about 3, and morepreferably from about ⅔ to about 1.5. The ratio of extinguishing toignition voltage is higher than for shallow cups and nearly an order ofmagnitude higher than if no cups were present. This permits the gasfeed-through 150 to extinguish any plasma formation and thereby mitigatedamage to any material within the base, in the electrically conductingcup itself, or adjacent O-ring.

[0032] The reduction in the electric field at the bottom of the cupsdepends on the aspect ratio (x) of the cups to effective openingdiameter 2 a. The field reduction may be estimated by an analyticcalculation, as shown in FIG. 6. In this calculation, a conducting planewith a circular aperture of radius a has an electric field E₀ appliedperpendicular to the plane. The electric potential behind the aperture(z<0) is given by:

φ(0,z)=E ₀ a[1+(z/a)arctan(−a/z)]/π  (1)

[0033] Differentiating to determine the electric field E_(z)=−∂φ/∂z, wefind (for z<0). J. D. Jackson, “Classical Electrodynamics,” 2^(nd) ed.(Wiley, NY, 1975), which is incorporated herein by references.

E _(z)(0,z)=E ₀[(a/z)/(1+(a/z)²)−arctan(−a/z)]π  (2)

[0034] Equation (2) may be used to estimate the ratio between theelectric field a distance −z behind the aperture and the applied field.It is probably an underestimate of the actual field reduction in the gasfeed-through 150 because of the field cancellation effect of theconducting sidewalls 185, which are not included in the analyticcalculation. A plot of E_(z)(0,z)/E₀ as a function of z/a is shown inFIG. 7. One can see that at a distance of −z/2a=1 behind the aperture,the electric field is about fifty times smaller than the field appliedto the plane. Thus, in a conducting cup with an aspect ratio of 1, theelectric field at the bottom of the hole will be about fifty timessmaller than the applied electric field. This significantly reduces theincidence of arcing or glow discharges in the gas passing through oradjacent to the passage 160 in the dielectric insert 155, and in the gapat the interface between the dielectric insert and the cup.

[0035] In a preferred version, as shown in FIGS. 3a and 3 b, thedielectric insert 155 comprises a porous, high surface area materialalong the walls of the passage 160 in the dielectric insert. This porousceramic has voids or pockets with an effective dimension d that arelinked to one another and supported by a web of ceramic material. Theporous material comprises a density fraction less than unity where unityrefers to the density of solid ceramic. It is believed that the porousmaterials limit the kinetic energy of any free electrons, therebyreducing the possibility of avalanche breakdown leading to plasmaformation in the passage 160 of the dielectric insert 155. Morespecifically, the small diameter pores have a small mean free pathlength inside the pore that serves to reduce the kinetic energy of thegas species traveling therethrough. In addition, the pore size of theporous ceramic represents the gap size d, from which the breakdownvoltage may be estimated from the Paschen curve. Preferably, the porousmaterial comprises pores that interconnect to form one or morecontinuous passages having small dimensions (i.e., diameter or length)which prevent an avalanche breakdown and plasma formation in the holes.Typically, the passageways through the pores have diameters ranging fromabout 1 μm to about 1 mm. The volume percent porosity of the porousmaterial is preferably from about 30 to about 90 volume %, and morepreferably from about 60 to about 80 volume %. Also, the porous materialpreferably comprises a surface area from about 20 cm²/g to about 300cm²/g.

[0036] Preferably, the porous ceramic material is formed within a sleeve200 composed of a substantially dense or substantially non-porousdielectric material, such as a ceramic or polymer, as shown in FIGS. 3aand 3 b. The sleeve 200 comprises a non-conducting media having a lowdielectric constant that is shaped and sized to fill the space betweenthe dielectric insert 155 and the surrounding shield to reduce plasmaformation therein. Also, the dense ceramic sleeve 200 reduces leakage ofgas from the passages 160 within the porous ceramic to the externalenvironment. In addition, the sleeve 200 assists in holding together theporous ceramic structure during the manufacturing process. The denseceramic of polymer sleeve 200 typically has a pore volume of less thanabout 10%.

[0037] More preferably, the gas feed-through 150 comprises a pluralityof sleeves that enclose the porous ceramic, including an inner sleeves200 a of dense ceramic and an outer sleeve 200 b of polymer, as shown inFIGS. 4a and 4 b. The dual sleeve 200 a,b facilitates manufacture of thegas feed-through 150 and also serves to form a good vacuum-tight sealaround the porous ceramic that can withstand low vacuum pressureswithout leakage of gas therethrough. It is also important to. minimizeany interfacial gaps between the porous ceramic, the dense ceramic innersleeve 200 a, and the polymer outer sleeve 200 b, and thereby ensuring atight fit between these components. It is further desirable to minimizethe gap between the polymer sleeve 200 b, the ceramic sleeve 200 a, andthe porous ceramic dielectric insert 155, and more preferably for thegap to be essentially zero, to reduce plasma formation in the gap. Oneway to achieve a tight fit between a polymer outer sleeve 200 b thatencloses a ceramic inner sleeve 200 a is to bore a hole in the polymerouter sleeve that has a slightly smaller diameter than the cross-sectionof the ceramic inner sleeve. After heating the polymer outer sleeve, theceramic inner sleeve 200 a is inserted into the hole to provide a tightfitting joint. In this version, preferably, the polymer outer sleeve 200b comprises polycarbonate or Lexan®, which is a brand having productdesignations GE-101 and GE-103, General Electric, Pittsville, Mass.

[0038] Another version of the gas feed-through 150, as shown in FIGS. 4aand 4 b, comprises a plurality of gas passages 160. This version isuseful for supplying heat transfer gas to multiple zones on a surface 40of an electrostatic chuck 25, as for example described in U.S. patentapplication Ser. No. 09/312,909, entitled “Chuck Having PressurizedZones of Heat Transfer Gas,” to Shamouilian, et al., filed on May 17,1999. In this version, two dielectric inserts 155 each comprise apassage 160 for the gas to flow therethrough. The two dielectric inserts155 are separated by a conducting cup-shaped metal shield that has acommon wall. A first sleeve made from Lexan™, comprises a casing havinga central stub 205 that projects outwardly relative to the passage 160,and also serves as a stop for the ends 210 of the electricallyconducting cup-shaped shield. The central stub 205 extends outwardly toincrease the path length for coupling of the electric field between theends 210 of the cup-shaped shields to reduce plasma arcing or glowdischarges across the ends of the cup-shaped shields. The dielectricinsert 155 comprises a porous ceramic insert having interconnected poresthat serve to form the passage 160. In addition, the inner and outersleeves 200 a,b comprise ceramic and polymer materials, respectively.

EXAMPLES

[0039] The following examples illustrate the principles of the presentinvention for reducing the incidence of plasma formation in the passage160 of a gas feed-through 150 for feeding gas to a chamber 10. However,the apparatus and method can be used in other applications as would beapparent to those skilled in the art, and the scope of the presentinvention should not be limited to the illustrative examples providedherein.

[0040] Four heat transfer gas feed-throughs 150 were tested on aneMxP+98 chamber having a high voltage RF and DC powered electrostaticchuck 25. Each gas feed-through 150 comprised a pair of electricallyconducting cup-shaped shields encircling the dielectric insert 155comprising porous ceramic. The porous ceramic had interconnected poreswhich formed the passage 160 for the heat transfer gas.

[0041] One of the gas feed-through 150 was modified to investigate theeffect of the depth of the cup-shaped shield on breakdown voltage. Thetests demonstrated that the gas feed-through 150 had adequate breakdownvoltages that exceeded about 2.7 KVp RF volts. Also the endplates of thecup-shaped shield were not damaged even when the shield arced in aplasma discharge in one experiment, and the only noticeable change tothe porous ceramic was a yellowing of the porous ceramic in regionswhere the plasma discharge occurred. This demonstrated the effectivenessof the gas feed-through 150.

[0042] The other gas feed-throughs 150 were modified and tested as shownin FIGS. 8 and 9. It should be noted that in a typical eMxP+chamber, themaximum voltage applied to the cathode electrode 50 is 4256V_(PP), whichis limited by the interlock trip. This RF voltage corresponds to a DCbias applied by autobias of −1850V_(dc). The combined peak voltagepresented to the He feed-through is therefore1-1850V_(dc)-2128V_(prf)1=3.98 KV, assuming the feed-through is equallysensitive to RF and DC components (which it may not be, as discussedbelow). Thus, the maximum peak voltage requirement has only an RFcomponent, equal to 2.13 KV_(prf). In these Figures, the upperhorizontal line represents the maximum RF+DC voltage for theeMxP+chamber, while the lower horizontal line represents the RFcomponent only, and represents the maximum voltage applied to theeMxP+98 chamber.

[0043]FIG. 8 shows a graphical summary of the performance of gasfeed-throughs 150 comprising dual gas feed pathways in porous ceramic,and having a electrically conducting cup 165 with an aspect ratio of−z/2a=1 and a 1 inch spacing between the endplates of the electricallyconducting cup. The data points below the line 300 indicate breakdownvoltages, and the data points above the line 300 show the correspondingplasma extinguishing voltages. The data points numbered 1 are for lessthan ten minutes of pumping and purging with He. The data point labeled1 is the breakdown voltage after the gas feed-through 150 has alreadyarced. The data points labeled 2 were taken on the same gas feed-through150 which were pumped and purged for several hours. All other datapoints were taken after the gas feed-throughs 150 had been pumped andpurged for at least 18 hours. Oil contamination was considered apossible reason for the drop in breakdown voltage because the portion ofthe porous ceramic that arced turned yellow in color. The data pointslabeled 5 and 6 correspond to the first set of porous ceramic gasfeed-throughs 150 which were ultrasonically cleaned in solvent todissolve any residual oil or other organic matter. Then the vacuumsystem was cleaned with solvent and changed to use a dry pump. There didnot appear to be any difference in performance between these data sets,from which one concludes that the drop in breakdown voltage over timewas due to out-gassing from the pores in the ceramic. It should be alsonoted that data points 1 through 6 are for one set of porous ceramicwhile 8 is for a different set having larger voids at their ends 210which also had relatively little influence on the performance of the gasfeed-through 150. This is due to the estimated factor of a fifty timeslower electric field strength at the bottom of the conductingelectrically conducting cup 165. An additional benefit of the lowerelectric field is the lack of damage to the endplates even at a lowaspect ratio. In this case, yellowing of the porous ceramic occurred dueto the exposure of the ceramic to ultraviolet light from the plasmadischarge. As a further affirmation of the reduction in electric fieldstrength at the bottom of the electrically conducting cups 165, it wasnoticed visually that approaching the breakdown voltage the plasma glowsonly in a small ½″ region between the endplates of the electricallyconducting cup. At slightly higher voltages, the plasma fills up theentire porous ceramic dielectric insert 155.

[0044] The top two data points numbered 6, in FIG. 8, have a verticalline through them that represents the DC component of voltage applied.In these two cases, a constant 3 KV_(prf) was applied and the negativeDC voltage was increased until breakdown occurred. Thus the DC voltagehas less effect on the breakdown voltage of this version of the gasfeed-through 150 than the peak RF voltage because of charge migration.In this design, the porous ceramic has pores with thin ceramic wallsthat are linked together. When a charge flows through the ceramic, thevoltage drops across each pore may decrease with the charge building upacross its ceramic walls. The DC electric field will then appear mostlyacross the ceramic rather than the pores containing the Helium gas. TheDC voltage is thus not effective in causing voltage breakdown.

[0045]FIG. 9 shows a summary of performance of the version of the gasfeed-through 150 comprising a porous ceramic dielectric insert 155 ofFIG. 4a and 4 b. In the Figure the data points below the line 305indicate breakdown voltages, and the data points above the line 305 showthe corresponding plasma extinguishing voltages. In these tests, thedepth of each of the cup-shaped shield was decreased by 0.2″, whichlowers the aspect ratio to −z/2a=0.75. Based on this scale, one wouldexpect the breakdown voltage to increase by the ratio of the gaps,1.4″/1″=40%. However, an observed disadvantage of increasing the gap isa yellow-brown staining due to a more intense plasma discharge. Becausethis may damage the O-ring or cause contamination, the larger aspectratio design was chosen. Also, the gas feed-through 150 had an outersleeve of Lexan™ which exhibited higher dielectric heating than manyplastics; however, it has good mechanical rigidity and low dielectricconstant 92.96 at 1 MHz. The dielectric heating can be estimated fromthe dissipation factor of Lexan, which is 0.0100 at 1 MHz. Thecapacitance of the He feed-through assembly was measured using an LCRmeter as 6 pF. Consequently, the capacitive reactance of the gasfeed-through is 1/ωC=1.96KΩ at 13.56 MHz. Then the equivalent seriesresistance R=0.01×1.96KΩ=19.6Ω. The dielectric heating may then beestimated from I²R=ω²C²V²R=12.4W at 2200V_(prf)—which is an acceptableamount of dielectric heating. The gas feed-through 150 becameperceptibly warm (perhaps 30° C.) to the touch after running for tens ofminutes at voltages near 3KV_(p). It was also observed that the shortercup version experienced less heating—which it should because itscapacitance is lower.

[0046] The electrically conducting cups 165 are found to minimize theelectric field at the interface between the electrically conducting cupand porous ceramic material and between the electrically conducting cupand the Lexan with O-ring seals. Another advantage is that the breakdownvoltage is independent of any imperfections in metal endplates, thepresence of features such as O-ring grooves 45 or the rings themselves,or imperfections like possible large voids at the ends 210 of the porousceramic dielectric insert 155. The reduced electric field at the twoendplates of the gas feed-through 150 makes the breakdown voltage of thegas feed-through very consistent and repeatable.

[0047] The present gas feed-through 150 provides a plasma extinguishingvoltage more than one-third the plasma ignition voltage. In addition,the gas feed-through 150 may also be scaled up in size to achievedesired ignition or extinguishing voltages.

[0048] While the present invention has been described in considerabledetail with reference to certain preferred versions, many other versionsshould be apparent to those of ordinary skill in the art. For example,the gas feed-through 150 can comprise alternative shapes andconfigurations of dielectric inserts 155 and metal shields. Therefore,the spirit and scope of the appended claims should not be limited to thedescription of the preferred versions contained herein.

What is claimed is:
 1. A substrate support comprising: (a) an electrode;(b) a dielectric layer covering the electrode, the dielectric layerhaving a surface to receive a substrate; (c) a gas feed-through toprovide a gas to the surface of the dielectric layer, the gasfeed-through comprising a conduit extending through one or more of thedielectric layer and electrode; (d) a dielectric insert in the gasfeed-through, the dielectric insert having a passage therein that allowsthe gas to be flowed therethrough; and (e) two opposing electricallyconducting cups around the passage in the dielectric insert.
 2. Asupport according to claim 1 wherein the electrically conducting cupsare shaped to reduce plasma formation in the gas feed-through.
 3. Asupport according to claim 1 wherein the electrically conducting cupseach comprise a jacket of metal-containing material.
 4. A supportaccording to claim 3 wherein the jacket comprises a cylindrical shape.5. A support according to claim 4 wherein the cylindrical shapecomprises an aspect ratio of width to height of from about ⅓ to about 3.6. A support according to claim 1 wherein the dielectric insertcomprises a porous ceramic having a pore volume of from about 30 toabout 90 volume %.
 7. A support according to claim 6 wherein thedielectric insert comprises a sleeve about the porous ceramic, thesleeve comprising a substantially non-porous ceramic or polymer.
 8. Asubstrate support comprising: (a) an electrode; (b) a dielectric layercovering the electrode, the dielectric layer having a surface to receivea substrate; (c) a gas feed-through to provide a gas to the surface ofthe dielectric layer, the gas feed-through comprising a conduitextending through one or more of the dielectric layer and electrode; (d)a dielectric insert in the gas feed-through, the dielectric inserthaving a passage therein that allows the gas to be flowed therethrough;and (e) two opposing metal-containing shields around the passage in thedielectric insert, whereby the shields are capable of reducing plasmaformation in the gas feed-through.
 9. A support according to claim 8wherein each shield comprises a cylindrical shape having an aspect ratioof width to height of from about ⅓ to about
 3. 10. A support accordingto claim 8 wherein the dielectric insert comprises a porous ceramichaving a pore volume of from about 30 to about 90 volume %.
 11. Asupport according to claim 10 wherein the dielectric insert comprises asleeve about the porous ceramic, the sleeve comprising a substantiallynon-porous ceramic or polymer.
 12. A method of processing a substrate ona support in a chamber, the method comprising: (a) passing a gas througha passage in the support; and (b) maintaining an electrical shieldaround a portion of the passage, whereby an incidence of plasmaformation of the gas passing through the passage may be reduced.
 13. Amethod according to claim 12 comprising charging an electrode adjacentto the passage to sustain a plasma in the chamber.
 14. A methodaccording to claim 13 comprising charging the electrode with an RFvoltage.